Dynamics (Hibbeler) (1).pdf
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Dynamics in Rigid Bodies fourteenth edition
Dynamics (Hibbeler) (1).pdf
•0 j'aime•530 vues
Signaler
Dynamics in Rigid Bodies fourteenth edition
Dynamics in Rigid Bodies fourteenth edition
Dynamics in Rigid Bodies fourteenth edition
![12.2 RECTILINEAR KINEMATICS: CONTINUOUS MOTION 19
12
12–22. A sandbag is dropped from a balloon which is
ascending vertically at a constant speed of 6 ms. If the bag
is released with the same upward velocity of 6 ms when
t = 0 and hits the ground when t = 8 s, determine the speed
of the bag as it hits the ground and the altitude of the
balloon at this instant.
12–23. A particle is moving along a straight line such that
its acceleration is defined as a = (-2v) ms2, where v is in
meters per second. If v = 20 ms when s = 0 and t = 0,
determine the particle’s position, velocity, and acceleration
as functions of time.
*12–24. The acceleration of a particle traveling along a
straight line is a =
1
4
s12 ms2, where s is in meters. If v = 0,
s = 1 m when t = 0, determine the particle’s velocity at s = 2 m.
12–25. If the effects of atmospheric resistance are
accounted for, a freely falling body has an acceleration
defined by the equation a = 9.81[1 - v 2
(10 -4
)] ms2
,
where v is in ms and the positive direction is downward. If
the body is released from rest at a very high altitude,
determine (a) the velocity when t = 5 s, and (b) the body’s
terminal or maximum attainable velocity (as t S ).
12–26. The acceleration of a particle along a straight line
is defined by a = (2t - 9) ms2
, where t is in seconds. At
t = 0, s = 1 m and v = 10 ms. When t = 9 s, determine
(a) the particle’s position, (b) the total distance traveled,
and (c) the velocity.
12–27. When a particle falls through the air, its initial
acceleration a = g diminishes until it is zero, and there-
after it falls at a constant or terminal velocity vf. If this
variation of the acceleration can be expressed as
a = (gv2
f) (v2
f - v2
), determine the time needed for the
velocity to become v = vf2. Initially the particle falls
from rest.
*12–28. Two particles A and B start from rest at the origin
s = 0 and move along a straight line such that
aA = (6t - 3) fts2
and aB = (12t2
- 8) fts2
, where t is in
seconds. Determine the distance between them when
t = 4 s and the total distance each has traveled in t = 4 s.
12–29. A ball A is thrown vertically upward from the top
of a 30-m-high building with an initial velocity of 5 ms. At
the same instant another ball B is thrown upward from the
ground with an initial velocity of 20 ms. Determine the
height from the ground and the time at which they pass.
12–30. A sphere is fired downwards into a medium with
an initial speed of 27 ms. If it experiences a deceleration of
a = (-6t) ms2
, where t is in seconds, determine the
distance traveled before it stops.
12–31. The velocity of a particle traveling along a straight
line is v = v0 - ks, where k is constant. If s = 0 when t = 0,
determine the position and acceleration of the particle as a
function of time.
*12–32. Ball A is thrown vertically upwards with a velocity
of v0. Ball B is thrown upwards from the same point with
the same velocity t seconds later. Determine the elapsed
time t 2v0g from the instant ball A is thrown to when the
balls pass each other, and find the velocity of each ball at
this instant.
12–33. As a body is projected to a high altitude above the
earth’s surface, the variation of the acceleration of gravity
with respect to altitude y must be taken into account.
Neglecting air resistance, this acceleration is determined
from the formula a = -g0[R2
(R + y)2
], where g0 is the
constant gravitational acceleration at sea level, R is the
radius of the earth, and the positive direction is measured
upward. If g0 = 9.81 ms2
and R = 6356 km, determine the
minimum initial velocity (escape velocity) at which a
projectile should be shot vertically from the earth’s surface
so that it does not fall back to the earth. Hint: This requires
that v = 0 as y S .
12–34. Accounting for the variation of gravitational
acceleration a with respect to altitude y (see Prob. 12–36),
derive an equation that relates the velocity of a freely
falling particle to its altitude. Assume that the particle is
released from rest at an altitude y0 from the earth’s surface.
With what velocity does the particle strike the earth if it is
released from rest at an altitude y0 = 500 km? Use the
numerical data in Prob. 12–33.](https://image.slidesharecdn.com/dynamicshibbeler1-230919163720-5b892ccf/75/dynamics-hibbeler-1pdf-44-2048.jpg?cb=1695141838)
![24 CHAPTER 12 KINEMATICS OF A PARTICLE
12
The car in Fig. 12–14a starts from rest and travels along a straight track
such that it accelerates at 10 ms2
for 10 s, and then decelerates at
2 ms2
. Draw the v9t and s–t graphs and determine the time t needed
to stop the car. How far has the car traveled?
EXAMPLE 12.7
t (s)
a (m/s2
)
(a)
10
2
10
A1
A2
t¿
t (s)
v (m/s)
(b)
100
10
v 10t
v 2t 120
t¿ 60
t (s)
s (m)
(c)
10 60
500
3000
s 5t2
s t2
120t 600
Fig. 12–14
SOLUTION
v–t Graph. Since dv = a dt, the v9t graph is determined by
integrating the straight-line segments of the a–t graph. Using the initial
condition v = 0 when t = 0, we have
0 … t 6 10 s; a = (10) ms2
;
L
v
0
dv =
L
t
0
10 dt, v = 10t
When t = 10 s, v = 10(10) = 100 ms. Using this as the initial
condition for the next time period, we have
10 s 6 t … t; a = (-2) ms2
;
L
v
100 ms
dv =
L
t
10 s
-2 dt, v = (-2t + 120) ms
When t = t we require v = 0. This yields, Fig. 12–14b,
t = 60 s Ans.
A more direct solution for t is possible by realizing that the area
under the a–t graph is equal to the change in the car’s velocity. We
require v = 0 = A1 + A2, Fig. 12–14a.Thus
0 = 10 ms2
(10 s) + (-2 ms2
)(t - 10 s)
t = 60 s Ans.
s–t Graph. Since ds = v dt, integrating the equations of the
v9t graph yields the corresponding equations of the s–t graph. Using
the initial condition s = 0 when t = 0, we have
0 … t … 10 s; v = (10t) ms;
L
s
0
ds =
L
t
0
10t dt, s = (5t2
) m
When t = 10 s, s = 5(10)2
= 500 m. Using this initial condition,
10 s … t … 60 s; v = (-2t + 120) ms;
L
s
500 m
ds =
L
t
10 s
(-2t + 120) dt
s - 500 = -t2
+ 120t - [-(10)2
+ 120(10)]
s = (-t2
+ 120t - 600) m
When t = 60 s, the position is
s = -(60)2
+ 120(60) - 600 = 3000 m Ans.
The s–t graph is shown in Fig. 12–14c.
NOTE: A direct solution for s is possible when t = 60 s, since the
triangular area under the v9t graph would yield the displacement
s = s - 0 from t = 0 to t = 60 s. Hence,
s = 1
2(60 s)(100 ms) = 3000 m Ans.](https://image.slidesharecdn.com/dynamicshibbeler1-230919163720-5b892ccf/75/dynamics-hibbeler-1pdf-49-2048.jpg?cb=1695141838)
![12.3 RECTILINEAR KINEMATICS: ERRATIC MOTION 25
12
The v–s graph describing the motion of a motorcycle is shown in
Fig. 12–15a. Construct the a–s graph of the motion and determine the
time needed for the motorcycle to reach the position s = 400 ft.
EXAMPLE 12.8
(a)
v (ft/s)
s (ft)
10
50
200 400
v 0.2s 10
v 50
(b)
200 400
s (ft)
a (ft/s2
)
10
2
a 0.04s 2
a 0
Fig. 12–15
SOLUTION
a–s Graph. Since the equations for segments of the v–s graph are
given, the a–s graph can be determined using a ds = v dv.
0 … s 6 200 ft; v = (0.2s + 10) fts
a = v
dv
ds
= (0.2s + 10)
d
ds
(0.2s + 10) = 0.04s + 2
200 ft 6 s … 400 ft; v = 50 fts
a = v
dv
ds
= (50)
d
ds
(50) = 0
The results are plotted in Fig. 12–15b.
Time. The time can be obtained using the v–s graph and v = dsdt,
because this equation relates v, s, and t. For the first segment of
motion, s = 0 when t = 0, so
0 … s 6 200 ft; v = (0.2s + 10) fts; dt =
ds
v
=
ds
0.2s + 10
L
t
0
dt =
L
s
0
ds
0.2s + 10
t = (5 ln(0.2s + 10) - 5 ln 10) s
At s = 200 ft, t = 5 ln[0.2(200) + 10] - 5 ln 10 = 8.05 s. Therefore,
using these initial conditions for the second segment of motion,
200 ft 6 s … 400 ft; v = 50 fts; dt =
ds
v
=
ds
50
L
t
8.05 s
dt =
L
s
200 m
ds
50
;
t - 8.05 =
s
50
- 4; t = a
s
50
+ 4.05b s
Therefore, at s = 400 ft,
t =
400
50
+ 4.05 = 12.0 s Ans.
NOTE: The graphical results can be checked in part by calculating slopes.
For example, at s = 0, a = v(dvds) = 10(50 - 10)200 = 2 ms2
.
Also, the results can be checked in part by inspection. The v–s graph
indicates the initial increase in velocity (acceleration) followed by
constant velocity (a = 0).](https://image.slidesharecdn.com/dynamicshibbeler1-230919163720-5b892ccf/75/dynamics-hibbeler-1pdf-50-2048.jpg?cb=1695141838)
![28 CHAPTER 12 KINEMATICS OF A PARTICLE
12 PROBLEMS
12–35. A freight train starts from rest and travels with a
constant acceleration of 0.5 fts2
.After a time t it maintains a
constant speed so that when t = 160 s it has traveled 2000 ft.
Determine the time t and draw the v–t graph for the
motion.
*12–36. The s–t graph for a train has been experimentally
determined. From the data, construct the v–t and a–t graphs
for the motion; 0 … t … 40 s. For 0 … t … 30 s, the curve is
s = (0.4t2) m, and then it becomes straight for t Ú 30 s.
t (s)
s (m)
600
360
30 40
Prob. 12–36
12–37. Two rockets start from rest at the same elevation.
Rocket A accelerates vertically at 20 ms2 for 12 s and then
maintains a constant speed. Rocket B accelerates at 15 ms2
until reaching a constant speed of 150 ms. Construct the
a–t, v–t, and s–t graphs for each rocket until t = 20 s.What is
the distance between the rockets when t = 20 s?
12–38. A particle starts from s = 0 and travels along a
straight line with a velocity v = (t2 - 4t + 3) ms, where t is in
seconds. Construct the v–t and a–t graphs for the time
interval 0 … t … 4 s.
12–39. If the position of a particle is defined by
s = [2 sin (p5)t + 4] m, where t is in seconds, construct the
s9t, v9t, and a9t graphs for 0 … t … 10 s.
*12–40. An airplane starts from rest, travels 5000 ft down
a runway, and after uniform acceleration, takes off with a
speed of 162 mih. It then climbs in a straight line with a
uniform acceleration of 3 fts2
until it reaches a constant
speed of 220 mih. Draw the s–t, v–t, and a–t graphs that
describe the motion.
12–41. The elevator starts from rest at the first floor of the
building. It can accelerate at 5 fts2
and then decelerate at
2 fts2
. Determine the shortest time it takes to reach a floor
40 ft above the ground. The elevator starts from rest and
then stops. Draw the a–t, v–t, and s–t graphs for the motion.
40 ft
Prob. 12–41
12–42. The velocity of a car is plotted as shown.Determine
the total distance the car moves until it stops (t = 80 s).
Construct the a–t graph.
t (s)
10
40 80
v (m/s)
Prob. 12–42](https://image.slidesharecdn.com/dynamicshibbeler1-230919163720-5b892ccf/75/dynamics-hibbeler-1pdf-53-2048.jpg?cb=1695141838)
![44 CHAPTER 12 KINEMATICS OF A PARTICLE
12
The chipping machine is designed to eject wood chips at vO = 25 fts
as shown in Fig.12–22.If the tube is oriented at 30° from the horizontal,
determine how high, h, the chips strike the pile if at this instant they
land on the pile 20 ft from the tube.
4 ft
O
30
y
x
20 ft
h
A
vO 25 ft/s
Fig. 12–22
SOLUTION
Coordinate System. When the motion is analyzed between points O
and A, the three unknowns are the height h, time of flight tOA, and
vertical component of velocity (vA)y. [Note that (vA)x = (vO)x.] With
the origin of coordinates at O, Fig. 12–22, the initial velocity of a chip
has components of
(vO)x = (25 cos 30) fts = 21.65 fts S
(vO)y = (25 sin 30) fts = 12.5 ftsc
Also, (vA)x = (vO)x = 21.65 fts and ay = -32.2 fts2
. Since we do
not need to determine (vA)y, we have
Horizontal Motion.
( S
+ ) xA = xO + (vO)xtOA
20 ft = 0 + (21.65fts)tOA
tOA = 0.9238 s
Vertical Motion. Relating tOA to the initial and final elevations of a
chip, we have
(+ c) yA = yO + (vO)ytOA + 1
2 actOA
2
(h - 4 ft) = 0 + (12.5 fts)(0.9238 s) + 1
2(-32.2 fts2
)(0.9238 s)2
h = 1.81ft Ans.
NOTE: We can determine (vA)y by using (vA)y = (vO)y + actOA.
EXAMPLE 12.12](https://image.slidesharecdn.com/dynamicshibbeler1-230919163720-5b892ccf/75/dynamics-hibbeler-1pdf-69-2048.jpg?cb=1695141838)
![12.6 MOTION OF A PROJECTILE 45
12
The track for this racing event was designed so that riders jump off the
slope at 30°, from a height of 1 m. During a race it was observed that
the rider shown in Fig. 12–23a remained in mid air for 1.5 s. Determine
the speed at which he was traveling off the ramp, the horizontal
distance he travels before striking the ground, and the maximum
height he attains. Neglect the size of the bike and rider.
(a)
SOLUTION
Coordinate System. As shown in Fig. 12–23b, the origin of the
coordinates is established at A. Between the end points of the path AB
the three unknowns are the initial speed vA, range R, and the vertical
component of velocity (vB)y.
Vertical Motion. Since the time of flight and the vertical distance
between the ends of the path are known, we can determine vA.
(+ c) yB = yA + (vA)ytAB + 1
2 act2
AB
-1 m = 0 + vA sin30(1.5 s) + 1
2(-9.81 ms2
)(1.5 s)2
vA = 13.38 ms = 13.4 ms Ans.
Horizontal Motion. The range R can now be determined.
( S
+ ) xB = xA + (vA)xtAB
R = 0 + 13.38 cos 30 ms(1.5 s)
= 17.4 m Ans.
In order to find the maximum height h we will consider the path AC,
Fig. 12–23b. Here the three unknowns are the time of flight tAC, the
horizontal distance from A to C, and the height h. At the maximum
height (vC)y = 0, and since vA is known, we can determine h directly
without considering tAC using the following equation.
(vC)y
2
= (vA)y
2
+ 2ac[yC - yA]
02
= (13.38 sin 30 ms)2
+ 2(-9.81 ms2
)[(h - 1 m) - 0]
h = 3.28 m Ans.
NOTE: Show that the bike will strike the ground at B with a velocity
having components of
(vB)x = 11.6 ms S , (vB)y = 8.02 msT
EXAMPLE 12.13
30
A
C
B
y
x
R
h 1 m
(b)
Fig. 12–23
(©
R.C.
Hibbeler)](https://image.slidesharecdn.com/dynamicshibbeler1-230919163720-5b892ccf/75/dynamics-hibbeler-1pdf-70-2048.jpg?cb=1695141838)
